12 ACCELERATION OF DEVELOPMENT IN FOSSIL CEPHALOPODA 



the development of Cerafites in the Germanic basin in the Middle Trias- 

 sic.* Here we have a group descended from Meekoceras of the Lowet 

 Triassic, and in general as far removed from that genus in specialization 

 as in time, but delaying in the Meekoceras stage, retaining until almost 

 mature many of the characters of that genus, and scarcely progressing 

 beyond it at maturity. 



Pavlow** has observed a similar phenomenon in the Ammonites of 

 the Lower Cretaceous of Russia. In both cases we have a beginning of 

 degeneration caused by unfavorable conditions of life in basins partly 

 shut off from the sea. A beginning of this stretching of the ontogeny is 

 seen in Trackyceras of the Upper Triassic (PL XV, figs, 13-16), where 

 stages that had long been obsolete in the group persist almost until ma- 

 turity, probably brought out by atavism. 



Arrest of Development. 



The next step in degeneration is arrest of development, where the 

 youthful stages are prolonged, and the form on reaching maturity finally 

 fails to reach the complete development of that species, and does not 

 attain to the complexity of its immediate ancestors. Such cases are 

 known in the Brachiopods, where in a living species sexual maturity may 

 be reached in stages much lower in specialization than the normal mature 

 form, so much so that these stages have even been described as independ- 

 ent genera. Such arrested forms may even give rise to a stock that never 

 reaches the full generic evolution of its ancestors.* 



Dr. C. E. Beecher** has aptly described this same phenomenon : "In 

 each line of progression in the Terebratellidae the acceleration of the 

 period of reproduction, by influence of environment, threw off genera 

 which did not go through the complete series of metamorphoses, but are 

 otherwise fully adult, and even may show reversional tendencies due to 

 old age ; so that nearly every stage passed through by the higher genera 

 has a fixed representative in a lower genus. Moreover, the lower genera 

 are not merely equivalent to, or in exact parallelism with, the early stages 



*See E. R. Philippi, Die Ceratiten des oberen deutschen Muschelkalkes. 

 Pal. Abhandlungen von Dames und Kayser, Bd. VIII, Heft 4, 1901, p. 359. 



**Le Cretace inferieur de la Russie et sa Faune. Nouv. Mem. de la Soc. 

 Imp6r. Nat. Moscou. Tome XVI, 1901, Part I, p. 62. 



*Fisclier and Oehlert, Brachiopodes, Mission Scientifique du Cap Horn, 

 p. 50-60. 



**Amer. Nat., vol. XXVII, 1893, p. 603. 



